1. Field of the Invention
The invention relates to guns utilizing a charge of liquid propellant which is bulk loaded into the combustion chamber of the gun. Control of the combustion process throughout the ballistic cycle is achieved by using charge position, charge loading density, chamber geometric configuration, propellant fill procedure, and igniter action to establish the desired hydrodynamic flow patterns which can couple properly with the combustion process.
2. Prior Art
Classical bulk loaded liquid propellant guns are nearly 100 percent fully loaded by volume with a propellant which is quite incompressible. A pyrotechnic igniter located near the breech end of the charge is used to initiate the combustion process. The ballistic cycle proceeds as follows:
Single or multiple hot gaseous jets spray from the igniter. The liquid pressure rises very sharply with the mass addition from the igniter because of the non-compliant liquid. Even though very little combustion has occurred, the high pressure caused by the igniter is sufficient to start projectile motion.
As the projectile moves, more volume is available for the combusting gases to expand into and the pressure drops because the amount of combustion established is not sufficient to maintain pressure while the projectile is moving. As the projectile moves down the tube, the light combustion gases in the breech accelerate the heavy liquid down the tube. This is an unstable flow condition and has been named the Rayleigh-Taylor instability. The light gases which can be accelerated down the tube more easily than the heavy liquid, try to achieve stability by changing places with the liquid. Multiple gas fingers penetrate into the liquid. As a hydrodynamic boundary layer is established in the tube, the penetrating gas fingers coalesce into a single central gas column which has been named a Taylor cavity. Throughout the Taylor cavity penetration process, the pressure continues to drop because insufficient combustion is occurring to maintain pressure with the volume expansion caused by projectile motion. After the Taylor cavity has penetrated to the base of the projectile, the liquid forms an annulus lining the tube wall and a gas core is established between the breech and the projectile. After penetration, the liquid is no longer accelerated at the same rate down the tube but rather the gases try to vent rapidly out the central core. Very high relative velocities are achieved between the gas core and the liquid annulus. This results in another classical flow phenomenon known as the "Kelvin-Helmholtz shear-layer instability". The disparate fluid velocities cause surface waves which result in droplets being stripped from the liquid surface and being entrained into the gas core. This mechanism of surface area augmentation is primarily responsible for achieving the high burn rates needed for successful ballistic performance. At the time the Taylor cavity penetrates to the projectile base, only about five percent of the liquid propellant has been burned. Only after complete penetration has occurred and the Helmholtz augment combustion is established does the pressure again begin to rise. This Helmholtz augmented burning continues until the liquid propellant charge is completely consumed by combustion.
While some control over the ignition process is possible, very little subsequent control is available for the Taylor cavity penetration and the Helmholtz burning. Fortunately these processes are somewhat self-controlling, as attested to by the thousands of successful bulk firings. As the projectile moves forwardly more rapidly, generating additional volume there behind, the Taylor cavity is able to penetrate faster and the shear-layer interface is able to elongate, thus greatly increasing the burn rate. Likewise, if the projectile moves forwardly more slowly, the burn rate stays at a modest level because the Taylor and Helmholtz mechanisms do not augment the reaction area as rapidly. Thus, high burn rates occur when they are needed and not when they cannot be tolerated.
Historically, the performance of bulk loaded firings has been plagued by a lack of sufficient controllability and repeatability. The most significant single opinion of prior researchers is that the non-repeatable ignition has been the primary cause of the non-repeatable muzzle velocity. Other causes for failure include excessively fine mixing, improper loading, questionable propellant composition, previously compromised materials, and delayed ignition. None of these causes is inherent to the bulk liquid propellant combustion process.
Examples of bulk loaded liquid propellant guns are found in U.S. Pat. No. 4,478,128, issued Oct. 23, 1984 to W. L. Black et al, and U.S. Pat. No. 4,160,405, issued July 10, 1979 to S. E. Ayler et al.
U.S. Pat. No. 4,269,107, issued May 26, 1981 to J. Campbell, Jr. shows a regenerative liquid propellant gun having a storage and pumping chamber aft of the piston and a combustion chamber forward of the piston. The inlets for propellant to the storage chamber are at an angle to the gun axis to provide a swirling flow which forces trapped bubbles out through a vent from the storage chamber.
U.S. Pat. No. 3,426,534, issued Feb. 11, 1969 to D. F. Murphy shows a rocket having a combustion chamber which is fed by a circular control chamber which has tangential fluid and gas inlets.